At present, the best room-temperature solid-state radiation spectrometers use CdTe, CdZnTe and HgI2 detectors. Other spectrometers, such as PbI2 and GaAs, also exist but are still at a research stage. All of these detectors are prone to errors in energy measurements, mainly due to incomplete charge collection of holes, an effect that falsifies the total recorded energy deposited in the detector. Indeed, the mobility-lifetime product for holes (.mu..sub.h .tau..sub.h) of these detectors is very low. Even for CdTe, which is one of the best quality detectors, this product is of the order of 10.sup.-4 cm.sup.2 /V. Thus, even at electric fields as high as 1000 V/cm, the mean free path of holes is no more than 1 mm or so. It is therefore impractical to use thick detectors and still obtain spectroscopic data at high efficiencies.
Many attempts were made to correct for incomplete charge collection. Most methods use correction functions which take into account the correlation between the rise time of the charge pulse created by a single photon interaction and the total charge collected. For example, N. Matsushita et al in Nucl. Instr. and Meth. Vol 179 (1981) p. 119 and in Nucl Instr and Meth. Vol 201 (1982) p. 433, empirically found such a correlation for Ge detectors and were able to correct the pulse height without any significant loss of efficiency. M. Richter and P. Siffert in Nucl. Instr. and Meth. Vol A 323 (1992) p. 529, used a similar technique and were able to correct empirically the pulse height spectra of several planar CdTe detectors. M. Saito in Patent Application No. 61-122818, Japan, 1986, used an electronic circuit to measure the transit time of electrons, i.e., the position of interaction, to correlate it with the total charge (electrons+holes) and to correct for charge trapping. Y. Eisen and Y. Horovitz in Nucl. Instr. and Meth. Vol A 353 (1994) p. 60, adopted another approach, in applying a theoretical correction function to account for incomplete charge collection in CdTe detectors.
However, in practice, any of the corrections of the type mentioned above for detectors with a high rate of hole trapping is prone to errors. For such detectors an approach alternative to total charge collection is therefore necessary.
It is the object of the present invention to provide a method and system for obtaining the spectroscopic data of a single photon interacting in a room-temperature solid state detector having significant hole trapping in which the errors discussed above are significantly reduced or eliminated.
In accordance with the present invention, the spectroscopic data is obtained by utilizing the electron induced current rather than obtaining it by correlating the risetime of the pulse, or any other measured parameter, with the shaped total charge (electrons and holes) as is done in the above cited references.